Typically, data is recorded with a constant linear velocity (CLV) in an optical disk, and the data recorded therein traces the same distance during any given time. Therefore, an optical disk system controlling a spindle motor rotating a disk recorded in this manner should operate according to CLV, so that low speed rotation is performed in the outer circumference of the disk and high speed rotation is implemented in the inner circumference.
For example, in a compact disk player (CDP) and its application apparatus, since the data is recorded with CLV, the angular velocity of the disk rotation changes depending on the relative radial position of the disk's pick-up. Whenever the pick-up unit for reproducing the data recorded in the disk is shifted to another position from some previous specific position, the speed of the spindle motor rotating the disk should be regulated accordingly.
In a conventional CDP, a data clock or a bit clock generated by a reproducing part after the speed is regulated is compared with a reference clock (for example, a system clock 4.3218 Mhz) generated within the optical disk system, in order to control the speed of the spindle motor by way of a phase difference. Thus the data clock at the desired position has the same frequency as the reference clock.
However, such a controlling method for the spindle motor has disadvantages in that it takes a long time to lock the frequency of the data clock to that of the system clock. Further, if the data clock is not properly extracted at the desired position because the speed deviates from a locking range of PLL (phase locked loop) in which the CLV can be adjusted, the motor speed is in danger of diverging.